In a liquid crystal display device, the liquid crystal is a substance that does not itself illuminate light. Instead, the liquid crystal relies on receiving light from a light source in order to display images and data. In a typical liquid crystal display device, a backlight module powered by electricity supplies the needed light.
FIG. 1 shows a typical backlight module 10. The backlight module 10 includes a reflecting sheet 11, a plurality of light sources 12 for emitting light rays, a guide plate 13, and an optical sheet 14. The light sources 12 are disposed between the reflecting sheet 11 and the guide plate 13. The guide plate 13 has a light incident surface 131 and a light-emitting surface 132 opposite to the light incident surface 131. Light rays emitting from the light sources 12 transmit into the guide plate 13 from the light incident surface 131 and then transmit out from the light-emitting surface 132, so as to improve the optical uniformity of the light rays. The optical sheet 14 is used to provide a more improved luminance and a better uniform brightness distribution. The optical sheet 14 can typically be one of an upper diffuser, a brightness enhancement film, and a lower diffuser. Partial light rays emitting from the light sources 12 transmit downwardly to the reflecting sheet 11, and then the reflecting sheet 11 reflects the light rays into the guide plate 13 so that the light rays can be utilized more effectively.
It is seen that, in the backlight module 10, most of the light rays emitting from the light sources 12 transmit in two general directions. In a first direction, the light rays emitting from light sources 12 transmit directly to the guide plate 13, and in a second direction, the light rays from light sources 12 transmit toward the reflecting sheet 11 and then reflected to the guide plate 13. The luminance of the light rays is in an inverse ratio to the traveled distance of light rays, thus, in the first direction, an area of the guide plate 13 closely adjacent to each light source 12 has a higher luminance than other areas further adjacent to each light source 12. In the second direction, most light rays reflected by the reflecting sheet 14 are reflected against the light source 12, thus, the reflecting light rays are mostly obstructed by the light source 12. Therefore, the light rays utilization is inefficient and the backlight module 10 is non-uniform. In order to improve the optical uniformity of the light rays, the distance between the light sources 12 and the guide plate 13 has to be increased, as a result, the size of the backlight module 10 is also increased and a liquid crystal display device using the backlight module 10 will also have a large size.
However, there is ongoing demand for backlight modules to provide an even more improved luminance and uniformity of illumination. A new light guide plate for a backlight module which can meet this demand is also desired.